One of the hallmarks of contemporary civilization is that each increment of technological progress almost invariably is accompanied by a similar increment of environmental regress. As the pace of technological advances quickens so does the march of environmental deterioration. The realization of environmental damage has occurred only relatively recently, so that present society finds itself burdened with the accumulated sins of the not-too-distant past. Many such burdens manifest themselves as toxic waste sites, i.e., geographical areas formerly used indiscriminately, or without recognition of inherent dangers, as dumps for waste materials and which now contain concentrations of one or more materials inimicable to the continued health of humans and of the environment generally.
A hallmark of current society is its acceptance of the undesirability of environmental degradation coupled with a determination to minimize it and reverse it wherever possible. A first step is the identification of potentially toxic sites and the materials which render such sites hazardous. A next step is the identification of methods and procedures which can render such sites at least environmentally neutral. Because the problems associated with toxic wastes are relatively new solutions for cleaning up such sites often are wanting or incomplete. The current surge in activity in developing adequate procedures for neutralizing toxic waste sites is a response to the new awareness of the undesirability of such dumps as well as an emerging determination to reverse the environmental trends of the past.
One kind of hazardous waste arises from the chromium roasting process, where chromium in iron-containing ore is oxidized to chromate to enable separation of the water-soluble chromate from insoluble ferric oxide. The residues from the aforementioned process contain chromate--more generally chromium as Cr(VI)--usually in a highly alkaline environment. When either acid or alkaline waters are neutralized, a very high concentration of ionic species, such as CaCl.sub.2, Na.sub.2 SO.sub.4, NaCl, NaOAc, CaOAc, are present in amounts up to about 10-15 wt. %. Under appropriate circumstances accumulated surface water can contain rather high concentrations of Cr(VI) in solution as chromate, with a concentrations of 25 milligrams per liter chromate and higher not being unusual. Since Cr(VI) is toxic at levels of about 5 ppm (as chromate) to humans, such surface water presents an immediate hazard to animals and an indirect hazard via the normal food chain to humans. Consequently, the Environmental Protection Agency has mandated that discharged waters must contain below 0.05 wt. ppm total chromium. Additionally, permeation of surface water through underlying strata threatens contamination of the subsurface water which could render wells unpotable and adversely affect marine life.
This application is directed toward immobilization of Cr(VI) dissolved in liquid aqueous wastes especially those containing a high concentration of ionic species. By "immobilization" is meant reducing the soluble Cr(VI) level, measured as chromium, to less than 0.05 part per million, preferably less than 0.01 ppm while transforming the remainder of the originally soluble Cr(VI) to an insoluble form of chromium which is highly resistant to leaching and which therefore minimizes environmental hazards. The method of our invention basically involves growing sulfate-reducing anaerobes in the liquid aqueous waste containing unacceptably high levels of chromium. What happens is that Cr(VI) is reduced to Cr(III) which precipitates as the hydroxide, an extremely insoluble species, within the pH range at which fermentation is conducted. The Cr(III) hydroxide settles as a solid and is removed or is mixed with other solid residues. When mixed with other solid residues the leachate therefrom is very low in Cr(VI), in fact sufficiently low as to pose virtually no environmental hazard. The naturally occurring reduction of Cr(VI) to Cr(III) by hydrogen sulfide produced by sedimentary bacteria previously has been noted by R. H. Smillie, K. Hunter, and M. Loutit, "Reduction of Chromium (VI) by Bacterially Produced Hydrogen in a Marine Environment," Water Research, 15, 1351-4 (1981). However, it is believed that sulfate-reducing anaerobic bacteria were considered to be unsuitable for treating industrial waste waters because of the inherent toxicity of chromium to microorganisms, as the following prior art indicates.
Revis et al. in U.S. Pat. No. 4,789,478 provide a brief discussion of many prior art patents pertaining to removal of heavy metals from waste waters using microorganisms, but omit reference to the reduction of Cr(VI).
In U.S. Pat. No. 4,522,723 Kauffman et al. disclose a process for reducing the concentration of water soluble ionic heavy metal species and sulfate ions in aqueous wastes. Although their principal interest appears to be in reducing uranium and molybdenum in mining waste waters, Kauffman et al. suggest the method can be employed with metal ions from many groups of the Periodic Table, including Group VIb, which contains chromium. However, there is no indication that the patentees' method had been applied to chromium-containing waters from industrial waste waters where the concentrations of chromium and other ionic species are very high. Such waters are particularly difficult to treat since they inhibit or are toxic to sulfate-reducing bacteria.
Romanenko et al. in U.S. Pat. No. 3,941,691 are consistent with Kauffman et al. and state that sulfates may be reduced to H.sub.2 S, which then reacts with soluble iron to form insoluble iron sulfides using sulfate reducing bacteria. However, they state that disulfovibro desulfuricans bacteria are not capable of reducing chromates and bichromates. Their invention resides in the selection of a microorganism which directly reduces the chromates and bichromates to chromium hydroxides without forming hydrogen sulfides.
Bopp, in U.S. Pat. No. 4,468,461 also discloses a new bacteria strain especially intended to remove chromates from waste water. In contrast with Kauffman et al. who used bacteria to produce H.sub.2 S which thereafter reduced the dissolved metals and presumably precipitated them as the metal sulfide, the microorganism used by Bopp directly reduced Cr.sup.+6 to Cr.sup.+3 and had resistance to the poisoning effect of dissolved chromium. In fact, the patentee states that his microorganism is capable of reducing Cr.sup.+6 up to 2000 ppm while other bacteria would not grow in concentrations more than 10-20 ppm. The microorganism is stated to be able to grow in either aerobic or anaerobic conditions, although aerobic conditions appear to be preferred. An organic reducing agent is required to satisfy the nutritional requirements of the microorganisms.
The present inventors have found that, contrary to the suggestions of the prior art, that it is possible to use sulfate-reducing bacteria to reduce Cr(VI) to Cr(III).